The present invention concerns a web former for a paper machine. This former comprises a lower-wire loop situated in conjunction with a head box of the paper machine and forming a single-wire, preferably substantially horizontal initial portion of a dewatering zone, on which water is removed through the lower wire by means of dewatering members and out of the web that is being formed. The former also comprises a top-wire unit which includes an upper wire guided by guide and web-forming rolls. The upper wire, together with the run of the lower wire, form a twin-wire dewatering zone in which the dewatering takes place substantially upwardly through the upper wire.
The former further comprises a first forming roll provided with an open hollow face and fitted inside the loop of the upper wire, the twin-wire forming zone beginning in connection with the first forming roll, and the twin-wire forming zone being curved upwardly over a certain sector of this forming roll. Moreover, the former additionally comprises a forming shoe which is situated after the forming roll and fitted inside the lower-wire loop, which guides the twin-wire dewatering zone. The forming shoe has a curved deck for guiding the lower-wire loop, with a center or centers of curvature of the deck being situated at the side of the lower-wire loop.
In prior art, twin-wire formers that are commonly used, so-called full-gap formers, the pulp is fed onto the wire part as a dilute suspension, after which the dewatering of the pulp forcibly begins directly or after a very short single-wire portion, in both directions or in the same direction as in the single-wire portion. A considerable proportion of the fillers and fines added to the pulp are also removed along with the water in this fashion. This naturally lowers the quality of the paper web and in particular deteriorates the properties that are supposed to be obtained for the paper web by means of the fillers. Simultaneous and intensive dewatering in two directions also readily causes weakening of the middle portion of the web which results in low internal bond strength.
In view of avoiding the drawbacks noted above, the Valmet Finnish Patent No. 50,648 suggests a twin-wire forming which is characterized by the initial single-wire part of the wire portion being sufficiently long so that as the dewatering gently takes place in this initial part, the pulp web has time to reach a degree of coating before the twin-wire portion so that the fibers can no longer be displaced relative to each other to a significant extent. The twin-wire portion is guided by a dewatering roll or by a dewatering box and is curved downwardly so that water is removed in this curved portion, in particular by the effect of a pressure zone produced by centrifugal force and by the tensioning between the wires through the upper wire and in the direction opposite to the direction in the single-wire initial portion. The primary objective therein is to reduce the removal of additives in the pulp web, such as the fillers as well as the fines in the web, and to also increase the internal bond strength of the paper to be produced.
Such prior-art, twin-wire formers in which no stationary dewatering members are used, usually have poor formation. It is not possible to produce such pulsations of the dewatering pressure to improve formation by means of these prior art devices. A further drawback has been that these formers have not included the possibility of regulating the ratios of the water quantities removed through the upper wire and through the lower wire.
Twin-wire formers are also known in the prior art, in which the dewatering is based almost exclusively on the use of stationary dewatering members. However, these formers involve the drawbacks of poor retention as well as rapid wear of the wires and high consumption of power.
In recent years, modernizations of fourdrinier machines have become common in which one or several top-wire units have been placed above the fourdrinier unit. By means of these top-wire units dewatering is produced upwardly, with the objective of both increasing the dewatering capacity and improving the formation and retention. On the other hand, an increased dewatering capacity permits an increased running speed of the paper machine. One objective has been to permit a lowering of the consistency of the pulp fed out of the head box, if necessary, which has favorable effects in and of itself. In some cases, old slow newsprint machines have been modernized to board machines by means of the top-wire units noted above, which produce thick qualities and require a high dewatering capacity without increasing the speed of the machine.
With respect to the prior art related to the present invention, reference is made to the following patents and patent applications: FI Pat. No. 80,648; FI patent application No. 820,742 (corresponding to U.S. Pat. No. 4,614,566); FI patent application No. 851,650; FI patent application No. 3240/70; FI patent application No. 822,705; U.S. Pat. No. 4,517,054; and U.S. Pat. No. 3,726,758.